Along with development of wireless multimedia services, requirements of people on high data rate and user experiences increasingly grow, so that higher requirements on system capacity and coverage of a conventional cellular network are made. On the other hand, requirements of people on knowing about interested persons or things nearby and proximity service gradually increase along with prevailing of applications such as a social network, short-distance data sharing and local advertising. A conventional Evolved NodeB (eNB)-centred cellular network has obvious limitations in terms of supporting to high data rate and proximity service, and in such a requirement background, a Device-to-Device (D2D) technology representative of a new direction of future development of communication technologies emerges. Application of a D2D technology may reduce a burden of a cellular network, reduce battery power consumption of a User Equipment (UE), increase data rate, improve robustness of a network infrastructure and well meet requirements on a high-data rate service and proximity service.
The D2D technology may work in an authorized frequency band or an unauthorized frequency band, and allows direct discovery/direct communication of multiple of UEs supporting a D2D function (i.e. D2D UEs) with a network infrastructure or without a network infrastructure. The D2D technology usually includes a D2D discovery technology and a D2D communication technology: the D2D discovery technology refers to a technology configured to judge/determine proximity between two or more D2D UEs (for example, within a range where D2D direct communication is allowed) or configured to judge/determine that a first UE is proximal to a second UE. Usually, the two or more D2D UEs may discover each other by sending or receiving discovery signals/information, and in a coverage of a cellular network, the cellular network may assist a D2D UE in D2D discovery; and the D2D communication technology refers to a technology configured to realize direct communication of a part or all of communication data between D2D UEs without a network infrastructure.
Mutual discovery and communication between UEs supporting a D2D function are not constant, but change along with relative movement of the UEs. As shown in FIG. 1, in a scenario with a coverage of a network infrastructure, data session communication is established between a UE1 and a UE2 supporting a D2D function, and a data stream is forwarded through a infrastructure of a cellular network. Along with gradual movement of the UE1 to a surrounding area of the UE2, the UE1 gradually gets close enough to the UE2 to support D2D discovery and D2D communication. In such a scenario, the data stream between the UE1 and the UE2 may be switched from a cellular communication mode into a D2D communication mode, thereby enjoying advantages of high rate, low delay, energy saving and the like of D2D communication. On the contrary, in a D2D communication process of the UE1 and the UE2, when the UE1 continues moving and gradually gets far away from the UE2, a data session between the UE1 and the UE2 is required to be switched from the D2D communication mode back into the cellular communication mode when a D2D communication link is unavailable. No matter switching from the cellular communication mode to the D2D communication mode or switching from the D2D communication mode to the cellular communication mode, a system requires to avoid users perceiving mode switching in a process of mode switching under a control of a network infrastructure, thereby ensuring service continuity.
From the above, inflexible mode switching in the related art may cause poorer service continuity.